As one method for imagewise exposing a photographic light-sensitive material, it is known to scan an original image and expose a silver halide photographic light-sensitive material according to the image signal obtained by the scanning to thereby form a negative or positive image in conformity with the original image. This is called the scanner method.
This scanner method includes two types: one for forming images with continuous gradation; and the other for forming halftone dot images. The latter halftone dot image-forming scanner method includes a so-called dot generator method using a halftone dot generator and a so-called screened scanner method for obtaining halftone dots by using a contact screen. As a light source for recording according to these scanner methods, a glow tube, a xenon lamp, a merucry lamp, a tungsten lamp, a light-emitting diode, etc., have been employed. However, all of these light sources have the practical defect that they provide a weak output and possess a short life. As a light source for the scanner methods removing these defects, there are coherent lasers such as Ne-He laser, argon laser, He-Cd laser or the like. These light sources can provide a high output, but have the defects that they are of large size, expensive, and require use of a modulation device and that they limit safelight of light-sensitive materials due to the use of fisible light, thus having poor handling properties.
On the other hand, scanners using semiconductor lasers have the merits that the light source is of small size, inexpensive, permits modulation with ease, and possesses a longer life then the above-described lasers, and that, since the semiconductor emits infrared rays, light-sensitive materials with sensitivity in an infrared region permits use of a bright safe light. Thus, such scanners provide improved handling properties of light-sensitive materials. However, the above-described excellent properties of semiconductor lasers have not been utilized due to the absence of light-sensitive materials having high sensitivity in an infrared region and having excellent preservability.
As commercially available light-sensitive materials with sensitivity in an infrared region, there is, for example, HIE135-20 made by Eastman Kodak Co. However, it is well known that these light-sensitive materials are unstable in sensitivity and require special caution for preservation thereof. For example, a catalogue of HIE135-20 indicates that the light-sensitive material should be stored in a freezer or refrigerator.
As one technique in producing photographic light-sensitive materials, an optically sensitizing technique is known which involves adding a certain kind of cyanine dye to a silver halide photographic emulsion to thereby expand the light-sensitive wavelength region of the light-sensitive material to a longer wavelength side. This technique is known to be applicable not only to a visible region but to an infrared region as well. For optical sensitization in an infrared region, sensitizing dyes which absorb infrared light are used. Examples thereof are described in, for example, Mees, The Theory of the Photographic Process, 3rd Ed. (Macmillan, 1966), pp. 198-201. With the abovedescribed infrared-sensitive materials, optical sensitivity, or sensitivity to infrared light, is desirably high, and less change in sensitivity during storage occur. For this purpose, many sensitizing dyes have so far been developed.
Examples of such dyes are described in, for example, U.S. Pat. Nos. 2,095,854, 2,095,856, 2,955,939, 3,482,978, 3,552,974, 3,573,921, 3,582,344, etc. However, those sensitizing dyes which are described in these patents do not provide enough sensitivity and preservability.
On the other hand, addition of a second specifically selected organic compound to a light-sensitive material in addition to the optically sensitizing dye sometimes remarkably raises the optical sensitivity. This is known as a supersensitizing effect. In general, addition of a second organic compound or an inorganic substance does not increase, but rather decreases, sensitivity. Therefore, the supersensitization can be said to be a specific phenomenon, and selection of the sensitizing dye and the second organic compound or inorganic substance to be combined with each other is remarkably restricted. Thus, an apparently slight difference in chemical structure can lead to such a great influence on the supersensitization effect that the supersensitizing combination is not predictable from chemical structure alone.
As the second organic compounds for supersensitization which are conventionally known, there are illustrated, for example, triazine derivatives described in U.S. Pat. Nos. 2,875,058 and 3,695,888, mercapto compounds described in U.S. Pat. No. 3,457,078, thiourea compounds described in U.S. Pat. No. 3,458,318, and pyrimidine derivatives described in U.S. Pat. No. 3,615,632. U.S. Pat. No. 4,011,083 describes effecting infrared sensitization using an azaindene compound and a desensitizing amount of an infrared-sensitizing dye.
However, the techniques described in these patents are still insufficient, though they truly increase infrared sensitivity and some of them provide somewhat improved preservability. Thus, a supersensitizing technique providing improved infrared sensitivity and improved preservability has been desired.
On the other hand, an emulsion in a solution state before being coated is generally liable to undergo change in sensitivity and fogging due to, particularly, removal, deposition or decomposition of the sensitizing dye. Such changes in photographic properties of an emulsion before coating is a critical problem in the production of light-sensitive materials. However, conventionally known stabilizers such as 1-phenyl-5-mercaptotetrazole are not effective for improving stability of an infrared-sensitizing dye-containing emulsion having been solated for coating. Therefore, a need exists to develop a technique which specifically improves solution stability with time of an infrared sensitizing dye-containing emulsion.